Divers surprised by iron age port

Archaeologists diving deep beneath the ferries and yachts criss-crossing Poole harbour have found startling evidence of the oldest working harbour in Britain, built centuries before the Roman invasion.

Timber pilings excavated from a deep layer of silt on the sea bed have been radio-carbon dated at 250BC, the oldest substantial port structures by several centuries anywhere on the British coast.

They suggest an iron age trading complex, with massive stone and timber jetties reaching out into the deep water channel, providing berths for the largest ocean going ships - raising the possibility of Greek and Roman traders making the journey from the Mediterranean to the Dorset coast.

"We assumed that the timbers would come back with a Roman date. It was a jaw dropping moment when we got the results of the tests," said Tim Darvill of Bournemouth University, who is head of the Poole harbour heritage committee.

The scale of the construction work was astounding, and implied a large, skilled and organised workforce. Two jetties have been traced, one with a surviving length of 45 metres, but probably originally the same length as the other 80 metre jetty. The surface was eight metres across, and smooth paved with shaped flagstones. The jetties were built up from an estimated 10,000 tonnes of rock and rubble, reinforced with hundreds of oak tree trunks, sharpened at one end so they could be rammed into the sea bed.

Local fishermen, sailors, and amateur divers have been involved in the hunt, and the team have put out an appeal for any sailor enraged by running aground or snagging lines on an uncharted obstacle to contact them.

Archaeologists had assumed that the earliest port structures in Poole harbour - the largest natural sea inlet in Europe - were Roman.

The alluring possibility for the archaeologists is that some of the ships themselves may remain, sunk into the deep layer of silt and as well preserved as the jetty timbers. "I'm not asking for much, a nice Graeco-Roman trader complete with its cargo would satisfy me," Prof Darvill said.

The finds prove that the Roman invaders did not found the harbour, but came to a place they knew well through generations of trade. The merchants were probably exporting local pottery, metal work, and shale jewellery, for which the area was renowned, and importing luxury goods such as amber, finer ceramics, and clay jars of costly produce such as wine and oil.